It is often desirable to ensure that a printed document has not been altered in some unauthorised manner from the time it was first produced. For example, a contract that has been agreed upon and signed on some date may subsequently be fraudulently altered. It is desirable to be able to detect such alterations in detail. Similarly, security documents of various sorts including cheques and monetary instruments record values that are vulnerable to fraudulent alteration. Detection of any fraudulent alteration is desirable. Further, it is desirable that such detection be performed automatically, and that the detection reveal the exact nature of the alteration. In addition to detection of fraudulent tampering with a document, it is desirable that such documents offer a visible deterrent to fraudulent alteration.
Various methods of deterring and detecting fraudulent alteration to documents have been proposed and used.
One class of methods in use before high quality colour scanners and printers became commonly available was to print important information such as monetary amounts in special fonts or with special shadows that were, at the time, difficult to reproduce. However, with modem printers and scanners, such techniques have become vulnerable to attack.
One known method of detecting alteration uses a 2D barcode printed on one part of a document page to encode (possibly cryptographically) a representation of some other portion, such as a signature area. This 2D barcode can be decoded and the resulting image compared by an operator to the area it is intending to represent to check for similarity.
A related body of work is the detection of tampering in digital images that are not subject to print/scan cycles. A number of “fragile watermark” techniques are known in this field, however these techniques are generally not applicable to tamper detection in printed documents because they cannot withstand the introduction of noise, Rotation, Scaling and Translation (RST), re-sampling, and local distortion that occurs in a print/scan cycle. Some of these techniques operate by replacing all or some of the least significant bits of pixels of an image with some form of checksum of remaining bits in each pixel.
A number of “semi-fragile” systems have also been described. These include systems that use cross-correlation to detect the presence of a lightly embedded shifted copy of a portion of the image. Another technique is to embed watermarks into image blocks, and then compare the detection strength of these watermarks to discern if any blocks have been altered. These systems tend to have less localisation ability as their detection ability improves, and as their localisation ability improves, they become more sensitive to noise and other distortions and so cannot be used to detect local changes in printed documents.
Other techniques use special materials to make alteration difficult. Such techniques include laminates covering the printed surface where damage to the laminate is obvious. However using special materials introduces production complexity, and is not applicable to plain paper applications. They are also not amenable to automatic detection.
An additional failing in many existing techniques is weak cryptographic security. In many cases, once the cryptographic algorithm being employed is identified, the identification leads directly to a subversion method to attack the identified method.
Another common failing of present techniques is the distribution of alteration detection information over wide areas of the page, or even areas completely separate to the image area to be authenticated (as in the barcode method above). This introduces problems if there is incidental soiling of the document in areas apart from the image area being authenticated. Many of these techniques cannot be used to authenticate the entire area of a document, so documents must be specifically designed to accommodate them.
A further class of techniques uses independent transfer of information about the original unaltered form of the document to the verification process. This could be as simple as a telephone call to a person with independent knowledge, and may extend to keeping a complete copy of the document in a secure location. Such techniques have many practical disadvantages because they require handling and storage of such independent information.